different circumstances--should all run to a certain extent
parallel with the systematic affinity of the forms which are subjected
to experiment; for systematic affinity attempts to express all kinds of
resemblance between all species.

"First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we
are to argue in a circle with respect to varieties in a state of Nature;
and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In
all other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels."--Pp. 276-8.

We fully agree with the general tenor of this weighty passage; but
forcible as are these arguments, and little as the value of fertility or
infertility as a test of species may be, it must not be forgotten that
the really important fact, so far as the inquiry into the origin of
species goes, is, that there are such things in Nature as groups of
animals and of plants, whose members are incapable of fertile union with
those of other groups; and that there are such things as hybrids, which
are absolutely sterile when crossed with other hybrids. For, if such
phenomena as these were exhibited by only two of those assemblages of
living objects, to which the name of species (whether it be used in its
physiological or in its morphological sense) is given, it would have
to be accounted for by any theory of the origin of species, and every
theory which could not account for it would be, so far, imperfect.

Up to this point, we have been dealing with matters of fact, and the
statements which we have laid before the reader would, to the best of
our knowledge, be admitted to contain a fair exposition of what is at
present known respecting the essential properties of species, by all who
have studied the question. And whatever may be his theoretical views, no
naturalist will probably be disposed to demur to the following summary
of that exposition:--

Living beings, whether animals or plants, are divisible into multitudes
of distinctly definable kinds, which are morphological species. They are
also divisible into groups of individuals,